Laserfiche WebLink
Mr. Carl Navarra Page 3 of 4 <br /> 7500 W. Eleventh Street February 23, 2010 <br /> Tracy, CA <br /> Contaminant Concentration Trends in MW-7 <br /> 10,000,000 <br /> (Free Product) <br /> 1.000,000- (Free Product) <br /> (Free Product) <br /> 100,000 <br /> a <br /> n <br /> a <br /> c <br /> 10,000 BMW-7TPWd <br /> (Free Product) MW-7 TPHg <br /> MW-7 Benzene <br /> m --(--MW-7 MTBE <br /> 1,000- <br /> r <br /> 0 <br /> V <br /> 100- 7V y�� V <br /> 10 <br /> (Non-Detect) (Non-Detect (Non-Detect) <br /> 1 <br /> Sampling Dates <br /> The first plot shows the concentration of MTBE in two on-site wells (MW-3 and MW-4) a down- <br /> gradient monitoring well (MW-6) on the 7501 W. Eleventh Street site, an intervening monitoring <br /> well, MW-13, located in the middle of Eleventh Street, and a down-gradient well (MW-18) <br /> approximately 560 feet from the former UST pit. The MTBE concentrations are generally highest <br /> in the two on-site wells and generally varied together over time, but do not display a strong <br /> declining trend. The concentrations in the down gradient well, MW-6, may be displaying a <br /> declining trend after a 4-year period of no sampling, but such a decline was not evident during <br /> the initial 5 years of monitoring. The MTBE concentrations also may be displaying a declining <br /> trend in MW-13, the well in the middle of Eleventh Street between on-site well MW-3 and off-site <br /> well MW-7. <br /> The TPHd, TPHg and benzene concentrations in MW-7 appear to display a declining trend <br /> during the last two monitoring events. The period during which TPH-g and benzene <br /> concentrations are `flat-lined' represent a time interval during which free product was present in <br /> the well casing and the plotted concentrations are not actual or theoretical measurements. The <br /> free product was not removed during that time and largely disappeared soon after removal <br /> began. <br /> The contaminant concentrations may have started declining during the last two monitoring <br /> events, but overall, the concentration data do not demonstrate well-established, strongly <br /> declining trends. A requirement for closure is that the contaminant concentrations and mass are <br /> declining and the time frame for obtaining background conditions is estimated. <br /> As noted in the EHD letter of 28 December 2008, methyl tert-butyl ether (MTBE) has been <br /> detected in the most down-gradient monitoring well (MW-19) during the last three monitoring <br /> events; while the detected concentrations have been quite low, the recent presence of MTBE in <br /> Site Closure Request Response Letter 0210 <br />